The magnitude of the explosion, now revised to 5.1, is significantly larger than North Korea’s previous nuclear tests in 2006 (4.3) and 2009 (4.7). Seismic scales are logarithmic, so 5.1 is much larger. The corresponding size of the explosion or "yield" is several kilotons, or thousands of tons of TNT — although we should treat all yield estimates at this point as rough approximations. (I’ve posted a long discussion on the perils of estimating yield based on the simple seismic data at ArmsControlWonk.com.)

The point is, this one is bigger. Although it’s not nearly so big as the modern thermonuclear weapons in the U.S. arsenal — which range from hundreds of kilotons to a megaton — you really wouldn’t want this dropped in your neighborhood.

More importantly, however, North Korea has announced that the device was a "miniaturized and lighter nuclear device with greater explosive force than previously." What this boils down to is a North Korean claim that this nuclear weapon will fit on a missile like the Nodong. Or maybe even the KN-08, which the North Koreans say over and over is intended for us. Like a valentine.

The question is, do we believe them? And if we do believe them, does it mean we have to think about this problem differently?

There are basically two approaches to these questions, which arise from differing interpretations of the rather small yields of the first two North Korean nuclear tests — a few hundred tons in 2006 and about two kilotons in 2009.

One view is to insist the North Koreans prove they are not terrible at building nuclear weapons. Here, I need to introduce you to a term of art: "simple fission device." This is a bulky nuclear weapon of the sort that the United States dropped on Nagasaki. That bomb weighed 4,000 kilograms (over four tons!) and contained six kilograms (about 13 pounds) of plutonium. The yield was about 20 kilotons. It was affectionately called "The Gadget," which is funny in precisely the opposite way that it’s amusing to name a Chihuahua "Bruiser." The massive Gadget would have been far too large to be delivered by a missile, if the United States had any at the time. No country has ever built a simple fission device and discovered it did not work. Hence, "simple," as in anyone can do it.

Until the North Koreans, that is. In this telling, the North Korean nuclear program, like the North Korean missile program, required multiple tries to achieve success: in this case, building a Gadget-sized simple fission device. The third time was a charm — but North Korea is still stuck with 1945 technology, wondering whether the damn thing will fit in the bomb bay.

The second view, to which I am inclined, is that we ought to take the North Koreans at their word. This view arises from the judgment that North Korea’s disappointing yields in 2006 and 2009 are not the result of technical incompetence so much as outsized ambition. The North Koreans tried to skip some steps and go directly to miniaturized devices.

The United States quickly reduced the mass and dimensions of its nuclear weapons, largely by making the process of implosion more efficient. When I visited the historical collection at the Atomic Weapons Establishment in Aldermaston, England, I was surprised to see a rather explicit description of the use of air pockets among the lenses to improve the efficiency of explosions. There are other tricks, too, like suspending or "levitating" the plutonium pit. (The physicist Ted Taylor elegantly illustrated the idea by asking, "When you hammer a nail, what do you do? Do you put the hammer on the nail and push?") The result was a steady reduction in size, without the loss of explosive power. One of the first miniaturized U.S. fission weapons, the Mark 7, weighed just 750 kilograms and used considerably less plutonium than the Gadget.

It’s no coincidence that the U.S. intelligence community, in the late 1990s, estimated that 650-750 kilograms was probably the best that North Korea could do without testing. The Mark 7 is used as a proxy for what a new nuclear state might attempt to build. China gave Pakistan a roughly comparable design, which Pakistan may have further shrunk, as well as sold to Libya and god knows who else. There are reports that Iran had access to a still more compact Soviet design.

The general view, however, is that miniaturized weapons are unreliable without testing. The question is whether the North Koreans attempted immediately to build a Mark 7-sized device, perhaps because they placed too much confidence in data acquired from the Soviet Union or Pakistan, or because they were just plain drunk on juche.

There is some evidence for this view, although it is admittedly circumstantial. In 2003, David Sanger reported that the United States noticed the North Koreans doing an unusual amount of testing with high explosives. The typical path toward shrinking nuclear weapons involves reducing the amount of explosives needed to compress a sphere of either plutonium or highly enriched uranium. The intelligence community, according to Sanger, concluded that North Korea was attempting to do better than a simple fission device, trying "to make nuclear warheads small enough to fit atop the country’s growing arsenal of missiles."

Then, in 2005, a certain North Korean official defected. "Kim Il-do" claimed to have worked for the Second Economic Committee of the National Defense Commission, which oversees North Korea’s nuclear and missile programs. Kim had some interesting things to say about North Korea’s nuclear weapons program. Here is a translation from the Korean article by my colleague, Hanah Rhee:

Mr. Kim, during an interview with [South Korea’s] National Intelligence Service, said that North Korea had manufactured a one-ton nuclear warhead with four kg of plutonium. He also said "North Korean scientists reported to Kim Jong-il that the weapon is functional, however, they have doubts about the level of performance that they manufactured." Mr. Kim said, "North Korea is not confident whether their large-sized nuclear weapons can actually work in practice which is why they are developing a smaller 500 kg version."

At the time, I was inclined to doubt the guy. That’s not how the United States, or anyone else, did it. And for good reason — if North Korea were to test such a weapon, the result would probably be a humiliating failure.

Then North Korea tested a humiliating failure.

North Korea told China to expect a four-kiloton yield. That’s broadly consistent with what one might expect from a device with only four kilograms of plutonium. The actual result was a few hundred tons’ worth of yield. Not something you’d want dropped on your neighborhood, to be sure, but also not something you’d be eager to explain to the Dear Leader. Suddenly, at least to me, Kim Il-do didn’t seem so crazy.

If you believe this second theory of North Korea’s strategic development, then North Korea isn’t incompetently following the U.S. or Soviet nuclear weapons development path. With fewer tests, North Korea is trying to move more quickly to larger, deliverable warheads based on the experience of others. Although I suspect I’m in the minority, I still believe that the North Koreans’ ultimate goal is a stockpile of missile-delivered thermonuclear weapons. Broadly speaking, I suspect this test is a step toward that end. There are many technical details that we don’t know at the moment — and may never know. We don’t know yet whether the device used plutonium, highly enriched uranium, or both, and we might never know. We also don’t know how North Korea made a bigger bang this time — whether the progressively larger explosions represent improvements in the existing approach or changes in the entire design philosophy.

But we should treat North Korea’s own statements about miniaturization seriously.

Not taking the North Koreans seriously compromises our ability to formulate a realistic approach to managing the problem. We have a tendency to see North Korea’s nuclear program as a vaguely ridiculous enterprise that exists largely to extort the United States. This view underestimates North Korea’s ambition with regard to its nuclear weapons program and the importance that the leadership in Pyongyang places on it. Kim Jong Un is not going to take the route Muammar Qaddafi did and give up his nuclear and missile programs — they are much too central to his regime’s ideology. (This is to say nothing of the terrible precedent that Qaddafi’s overthrow set. After learning about the last few minutes of his life, I sat gingerly for a week.)

If you want to know how the North Koreans think about their nuclear weapons, watch these subtitled clips from the North Korean propaganda film, The Country I Saw. The complete, verifiable, and irreversible disarmament of North Korea, as it were, will require a comprehensive settlement of the questions arising from the Korean War. Let’s not hold our breath.

Being realistic about the dim prospects for disarmament should include an appreciation that things can get worse. A lot of pundits and politicos treat nuclear weapons as an either/or proposition. You have ‘em, or you don’t. A classic example of this sort thinking was a comment by Colin Powell, who argued in 2002 that the collapse of the 1994 Agreed Framework, which had frozen North Korea’s nuclear program, was harmless because the North already had nuclear weapons. "What are they going to do with another two or three nuclear weapons?" he asked on one of the Sunday talk shows. (He seems to have done five that day, giving variations on theme of "so what?") A small stockpile of reliable warheads small enough to ride a missile is clearly worse than a single one-ton device that doesn’t work. And, if we do nothing, whatever stockpile North Korea has in 10 years’ time could easily be worse than its current arsenal.

We ought to be careful about encouraging the North Koreans to prove it to us. I understand, and support, the Obama administration’s insistence on the talking point that we do not accept North Korea as having the formal status of a nuclear state. But making it clear that Pyongyang’s isolation cannot end with a nuclear-armed North Korea does not require insisting that it demonstrate each measure of capability. In the mid-1960s, the Chinese made some interesting choices about how to ensure the neighbors took them seriously. One was to mount a miniaturized nuclear weapon on a DF-2 missile and fire it 1,200 km across China. That was the fourth Chinese nuclear test. (The design from that test, which used highly enriched uranium, was the one that later showed up in Pakistan and Libya. One presumes more copies are floating around.) The North Koreans seem unwilling to test in the atmosphere for a variety of reasons, so I suspect this is an unlikely precedent. But it is worth keeping in mind that things could be much, much worse. There is value to simply managing a problem, especially when there are no better options.

We can all recite a list of options to manage this particular headache. Prominent among them: partial agreements to freeze fissile-material production, nuclear testing, and rocket launches. These steps will not eliminate North Korea’s capabilities, but now it’s my turn: So what? What better plan can you imagine? I think back to former diplomat Bob Gallucci’s frustration with critics of the much-maligned 1994 freeze, admittedly an unsatisfying approach:

"When I came back with the Agreed Framework deal and tried to sell it," he said, "[Many people] hated the idea of trying to solve this problem with a negotiation.

That’s because there isn’t a goddamn answer. There is no better plan that stops the North Korean nuclear program. The North Koreans will probably cheat and we will be rewarding bad behavior. So f’ing what? Unless, of course, you have a better idea. Then, by all means, be my guest.